1. Field of the Invention
The present invention relates to a semiconductor device including thin film transistors (TFTs), and a method for manufacturing the same. More particularly, the present invention relates to a semiconductor device including thin film transistors in which the semiconductor layer (active region) is formed from a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film, and a method for manufacturing the same. The present invention can suitably be used in active matrix liquid crystal display devices, organic EL display devices, contact image sensors, and three-dimensional ICs, and other suitable devices.
2. Description of the Related Art
In recent years, attempts have been made in the art to form high-performance semiconductor elements on an insulative substrate such as a glass substrate or an insulating film, aiming at realization of liquid crystal display devices and organic EL display devices having larger sizes and higher resolutions, contact image sensors operating at higher speeds with higher resolutions, three-dimensional ICs, etc. Particularly, a type of liquid crystal display device having the pixel section and the driving circuit on the same substrate is finding its use in various household appliances, in addition to a monitor of a personal computer (PC). For example, liquid crystal displays are used as television sets, replacing CRTs (Cathode-Ray Tubes), and front projectors are used for home entertainment applications such as for watching movies and for playing video games. Thus, the market for liquid crystal display devices has been growing at a remarkable rate. Moreover, system-on-panel devices have been developed actively, in which logic circuits such as a memory circuit and a clock generation circuit are formed on a glass substrate.
Displaying high-resolution images means an increase in the amount of data to be written to pixels, and the data needs to be written within a short time. Otherwise, it is not possible to display a moving picture that has a very large amount of data for high-definition display. Therefore, TFTs used in a driving circuit are required to operate at a high speed. In order to achieve high-speed operations, there is a demand for forming the TFTs using a crystalline semiconductor layer having a desirable crystallinity, with which it is possible to obtain a high field-effect mobility.
The present inventors have developed a method for obtaining a desirable crystalline semiconductor layer on a glass substrate. In this method, a metal element capable of promoting crystallization is added to an amorphous semiconductor layer, which is then subjected to a heat treatment. With this method, a desirable semiconductor film having a uniform crystal orientation can be obtained through a heat treatment performed at a lower temperature and for a shorter time than other conventional methods.
However, when a silicon film crystallized with a catalyst element is used as is for the semiconductor layer of a TFT, the TFT will have an abrupt increase in the off-state current. The catalyst element irregularly segregates in the semiconductor film, and the segregation is significant at crystal grain boundaries. It is believed that the segregation of the catalyst element creates leak paths for a current, resulting in the abrupt increase in the off-state current. Therefore, after the formation of the crystalline silicon film, it is necessary to reduce the catalyst element concentration in the semiconductor film by moving the catalyst element out of the semiconductor film. The step of removing the catalyst element will be hereinafter referred to as a “gettering process”. Moreover, the action of moving (attracting) the catalyst element will be hereinafter referred to as “gettering action”, and the element capable of attracting the catalyst element will be hereinafter referred to as “gettering element”.
Various types of gettering processes and methods have been proposed in the art.
For example, Japanese Laid-Open Patent Publication No. 8-213317 discloses a technique of forming an amorphized region in a silicon material that has been crystallized by using a catalyst element, and subjecting the silicon material to a heat treatment so that the catalyst element is moved (gettered) into lattice defects in the amorphized region. Japanese Laid-Open Patent Publication No. 8-213317 attempts to simplify the manufacturing process by using the source/drain region of the TFT as the gettering region. However, the method requires an additional step of activating the source/drain region with laser light, or the like, since an amorphous region as is cannot function as a source/drain region.
Japanese Laid-Open Patent Publication No. 8-330602 discloses a method that utilizes the gettering action of phosphorus. In the method, an active region (semiconductor layer) is formed by using a silicon material crystallized by using a catalyst element, and the source/drain region of the n-channel TFT is doped with phosphorus while the source/drain region of the p-channel TFT is doped with phosphorus and with boron at a higher concentration than that of phosphorus. Then, the structure is subjected to a heat treatment so as to getter the catalyst element to the source/drain regions.
Japanese Laid-Open Patent Publication No. 10-270363 discloses a technique of selectively introducing a group VB element such as phosphorus into a portion of a silicon material that has been crystallized by using a catalyst element, and subjecting the silicon material to a heat treatment at a temperature that does not exceed the deformation point of the substrate so that the catalyst element is moved (gettered) into the region where the group VB element has been introduced (gettering region). According to Japanese Laid-Open Patent Publication No. 10-270363, the gettering region is formed outside the island-shaped semiconductor layer (TFT active region), and the gettering region is removed after the gettering heat treatment. Then, the region where the catalyst element concentration has been lowered (hereinafter referred to also as “lightly-doped region”) is used to form the active region of the semiconductor element (TFT).
The conventional gettering processes, including those disclosed in the three publications mentioned above, have various problems such as the need for additional steps for the gettering process, which complicates the manufacturing process and increases the manufacturing cost. One solution to this problem is to remove the catalyst element from the channel region by moving the catalyst element into regions of the semiconductor layer that are to be the source/drain regions, instead of removing the catalyst element from the entire semiconductor layer of the TFT. However, various studies by the present inventors revealed that the techniques disclosed in the publications mentioned above have other problems as follows.
When a group VB element capable of moving a catalyst element is introduced into a region of a silicon film, as in Japanese Laid-Open Patent Publication No. 8-330602 and Japanese Laid-Open Patent Publication No. 10-270363, the solid solubility of the region for the catalyst element is increased, thereby gettering the catalyst element (the first type of gettering action). In contrast, in Japanese Laid-Open Patent Publication No. 8-213317, a catalyst element is gettered by utilizing crystal defects of an amorphous region as localized segregation sites for trapping the catalyst element (the second type of gettering action). Since the free energy of the catalyst element is lower in an amorphous region than in a crystalline region, the catalyst element is likely to diffuse into the amorphous region.
In order to increase the gettering capability of a gettering region, it is necessary to sufficiently affect the first type of gettering action and the second type of gettering action. However, it is difficult to achieve in a source region or a drain region of a thin film transistor. An effective way of increasing the gettering efficiency is to introduce a large amount of a gettering element into a source region and a drain region, which function as gettering regions, while amorphizing the regions. However, it will then significantly increase the resistance of the source region and the drain region so that they will no longer function as a source region and a drain region.
When a large amount of a gettering element is ion-implanted into a region of a crystalline semiconductor layer, the crystalline structure of the region is destroyed and the region is amorphized. The amorphization proceeds starting from the upper surface of the semiconductor layer, and when it reaches the lower surface of the semiconductor layer, the crystallinity of the semiconductor layer can no longer be recovered even by a heat treatment. With conventional methods in which the source region and the drain region are used as gettering regions, the crystallinity of the doped regions needs to be recovered at least to some degree by a subsequent heat treatment so as to reduce the resistance of the regions. Thus, with these methods, the dose cannot be increased beyond a maximum level at which the crystallinity can later be recovered, and it is difficult to increase the gettering efficiency by implanting a large amount of a gettering element. On the other hand, a sufficient gettering capability cannot be obtained with a small dose of a gettering element. Thus, with these methods, the most difficult problem is how to appropriately control the dose of a gettering element. When such a technique was actually applied to a liquid crystal display device integrated with a driver (driving circuit), the source region and the drain region in some regions of the substrate were amorphized and the resistance thereof was increased, thereby resulting in defective TFT on-state characteristics and thus a driver defect. In some other regions, the dose of the gettering element was insufficient, resulting in insufficient gettering, thereby increasing the off-state leak current, resulting in line defects and point defects. Thus, these methods have a very small process margin, and are difficult to use for mass production.
The method disclosed in Japanese Laid-Open Patent Publication No. 8-213317 requires an additional step of activating the source/drain region with laser light, or the like, since an amorphous region as is cannot function as a source/drain region. However, a laser irradiation apparatus is expensive and complicated in structure, and has a poor maintainability, thereby increasing the manufacturing cost and reducing the production yield. Moreover, with only the laser irradiation process, crystal defects occurring at the junction between the channel region and the source/drain region cannot be recovered, thereby resulting in a poor reliability and an increase in the off-state leak current.
Moreover, when the present inventors actually produced TFT elements in an experiment using these conventional methods, defective TFTs with abnormal levels of TFT off-state leak current at a defect rate on the order of 0.1% were produced. An analysis has confirmed that the defective TFTs had masses of a silicide of the catalyst element at the junction between the channel region and the drain region. Thus, with the conventional techniques of the publications mentioned above, the catalyst element is not gettered sufficiently. Even though these conventional techniques are capable of producing some high-performance TFTs, with such high defect rates and poor reliabilities, they cannot be used for mass production. The problem with insufficient gettering of the catalyst element and its resultant problems were not recognized previously in the art.
The increase in the TFT off-state leak current due to the presence of a catalyst element is caused primarily by segregation of the catalyst element at the junction between the channel region and the drain region. With a method in which the source region and the drain region are used as gettering regions, the junction between the channel region and the source/drain region is also the junction between the gettering region and the non-gettering region. Therefore, with such a gettering method, it is difficult to completely prevent the increase in the TFT off-state leak current due to the presence of a catalyst element.
In addition, according to Japanese Laid-Open Patent Publication No. 8-213317, the amorphous gettering regions (the source region and the drain region) are eventually crystallized. Then, the gettering effect will be small thereafter, whereby the catalyst element that has been moved away in a heat treatment may later come back in a reverse flow (diffusion into the channel region) in a subsequent step. Moreover, even if such a reverse flow of the catalyst element is prevented from occurring during the manufacturing process, heat is generated in no small measure from driving the TFT, and the catalyst element that has been once moved into the gettering region may come back to the channel region in a reverse flow when driving the TFT, thus resulting in a reliability problem. Therefore, in a case where the gettering region is provided in the active region (semiconductor layer) of the TFT, it is preferred that the gettering capacity of the region is kept even after the completion of the TFT so as to keep the same level of gettering capability as that during the gettering process.
Furthermore, with such a method as that of Japanese Laid-Open Patent Publication No. 10-270363, in which a gettering region is formed outside of an island-shaped semiconductor layer (TFT active region) and is removed after gettering the catalyst element thereto, no gettering region is present in the final TFT, whereby the catalyst element may come back to the channel region in a reverse flow when driving the TFT, thus resulting in a reliability problem.
Moreover, in the method of Japanese Laid-Open Patent Publication No. 10-270363, a gettering region is formed outside of an island-shaped semiconductor layer, thereby requiring additional steps of forming a mask, implanting a gettering element, performing a gettering heat treatment, etc. Moreover, since the distance required for gettering is relatively long, the gettering heat treatment may take a long time or may fail to provide sufficient gettering efficiency.